The present invention relates to an image forming apparatus using an electrophotographic developing method, and more particularly, to a method of automatically changing the transfer voltage depending upon a path of the recording media.
In order to achieve optimum image density on a piece of recording media in the electrophotography process, the thickness of the document must be considered. If a thick document, such as a postcard, transparency, envelope or, label is to be printed, the transfer voltage needs to be increased in order to achieve a quality result. Contemporary practice in the art, represented by U.S. Pat. No. 5,486,903 for an Image Forming Apparatus With Paper Thickness Detector to Kanno et al., endeavors to construct an image forming apparatus by using a capacitive thickness detector for the recording medium. The recording medium is passed between the plates of a capacitor biased at a fixed voltage, and the capacitance is then measured. From the value of the measured capacitance, the thickness of the printing medium may be determined. After the thickness of the printing media is determined, memory is consulted to determine the appropriate transfer voltage that needs to be applied to the transfer roller to obtain optimum image density on the printing media. Thus, an appropriate bias voltage is applied in the electrophotographic process depending on the gauged thickness and electrical resistance of the printing media. Additional examples of recent efforts in the art are found in U.S. Pat. No. 5,530,522 for an Image Forming Apparatus With Controlled Transfer Voltage to Tsunerni, U.S. Pat. No. 5,455,664 for an Electrophotographic Printer For Transferring Images on Different Sized Print Medium And Transferring Method Of The Same to Ito et al, U.S. Pat. No. 5,374,981 for an Electrostatic Recording Apparatus Capable of Maintaining Constant Gap Between Flexible Recording Electrodes and Opposite Electrode By Flexible Recording Electrodes to Yamamoto et al., U.S. Pat. No. 5,504,565 for an Image Forming Apparatus Having Transfer Voltage Timing Control to Tomiki et al., and U.S. Pat. No. 5,250,999 for an Image Forming Apparatus Having Transfer Voltage And Process Speed Control to Kimura et al. In Tsunemi ""522 and Ito et al. ""644, the transfer voltage is adjusted depending on the width of the recording media, in Yamamoto et al. ""981 and in Tomiki et al. ""565, the transfer voltage is adjusted depending on the thickness and the size of the recording media, while in Kimuranet, et al. ""999 the transfer voltage is adjusted according to the type of recording medium.
Other efforts in the art sought to provide electrophotographic copiers that allow for and contain numerous input ports for inserting the recording media. For example, U.S. Pat. No. 5,237,378 for a Copier/Printer Employing a Roll Media Feed Apparatus and Dual Functions Sensors to McEwen discloses a copier/printer that has three inputs to accommodate for three different recording media. Each input has a sensor to detect whether or not that particular recording medium is being conveyed to the printer in order to detect jamming in the photocopying machine. Thus, McEwen ""378 uses sensors on each recording medium input feed path to aid in the process of the printing operation.
Another example is found in U.S. Pat. No. 5,530,522 for an Image Forming Apparatus With Controlled Transfer Voltage to Inage et al. which uses an electrophotographic machine containing a plurality of recording medium trays and recording medium conveyer paths that merge into a single conveyer path before entering the printing unit. Inage et al. ""489 has a controller which dictates when a sheet of recording medium is to be dispensed, and from which tray the sheet is to be dispensed. The controller decides the sequence and the quantity and the timing for dispensation of the recording medium from each of the plurality of feeding trays. Inage et al. ""489 seeks to overcome the situation of having two subsequent sheets of recording medium overlap during the conveying process. Therefore, in Inage et al ""489, it is essential to know from which feeding tray each recording sheet originates from so that the timing between each sheet of recording medium does not go awry.
U.S. Pat. No. 5,321,485 for a Printing Apparatus Having Manual Sheet Feeding And Document Reading Capabilities to Nukaya discloses a manual feed electrophotographic printing apparatus containing two paths by which the paper may be transported. In addition, a CPU is connected to the electrophotographic machine. A path selector can be set to either a first position or a second position, where the first position ejects the document and the second position guides the sheet to a printer. If the document is ejected, a sensor senses this ejection and sends a signal to the CPU accordingly. If the document is sent to the printer, another sensor senses the presence of the document in the printer and sends a signal to the CPU. Thus, the electrophotographic operation depends upon signals received from sensors along the path taken by the recording medium. I have found that designs such as those represented by McEwen ""378, Inage ""489 and Nukaya ""485 lack an ability to adjust the transfer voltage based on any of these sensed signals.
U.S. Pat. No. 5,444,524 for a Method and Apparatus For Controlling a Print Engine Of A Page Printing Device to Lee endeavors to control the electrophotographic printing process, but lacks ability to accommodate multiple trays of a recording medium. I have discovered that there is a need for an electrophotographic machine that varies the transfer voltage on the basis of the path taken by the sheet of recording medium. In other words, it is my belief that what is needed is an electrophotographic process that automatically adjusts process parameters on a basis of the tray from which the sheet of recording medium originates, so that optimum image density is achieved.
Accordingly, it is an object to provide an improved electrophotographic photocopier.
It is another object to provide an electrophotographic photocopying machine that optimizes image density by automatically adjusting the transfer voltage in accordance with the tray or feeding unit from which the sheet of recording medium originates.
It is still another object to sense the path of conveyance of a sheet of recording medium and to adjust the transfer voltage accordingly to achieve optimal image quality.
It is yet another object to provide an engine controller for the electrophotographic process that receives signals from sensors disposed to detect the path of a sheet of recording medium and to generate a representative control signal to a transfer voltage generator based on the representative signals from sensor signals.
These and other objects may be attained according to the principles of the present invention by recognizing that if an electrophotographic, device has both an automatic feeding cassette and a manual feeding tray, it may be possible to print quality images on a variety of recording media by sensing the path of the recording media instead of measuring its thickness. In other words, by being able to detect from which input cassette the recording media originates, the transfer bias may be appropriately adjusted in order to optimize image density and image quality. This may be implemented with a transfer voltage generator that generates a first transfer voltage when a first control signal is received from the engine controller, and outputs a second transfer voltage of a different magnitude than the first transfer voltage when a second control signal is received from the engine controller so that one optimal transfer voltage is applied when the recording medium originates from the manual feeding tray and that a relatively different but nonetheless optimal transfer voltage is applied to the electrophotographic process when the recording medium originates from the automatic feeding cassette.